1. Field of the Invention
The present invention relates generally to filter systems for removing particulate solids from an air stream, and in particular to filter systems for continuously removing resinated wood particles and fibers from an air stream.
2. Description of the Related Art
Filter systems used for continuously removing small particulate solids, such as wood particles and fibers, from an air stream include cyclone separators, bag house filter systems, and porous static filter elements. Each of these types of system has its advantages and disadvantages. Cyclone separators are simple and inexpensive to build, install and maintain. Cyclones' main utility is to separate a single fluid stream having entrained particles over a range of sizes into two or more air streams, namely an underflow stream containing the particles over a predetermined size, and an overflow stream containing the particles under a predetermined size. Cyclones are limited however in their ability to separate very small sized particles from an air stream. Therefore if the cyclone overflow stream is to be discharged to the environment, further treatment to remove a portion of the small particles from the cyclone overflow stream may be required.
Bag houses on the other hand are generally very effective at removing a high percentage of entrained particles down to a very small size from an air stream. Unfortunately, bag house systems capable of filtering large volume air streams are large, complex, and involve high maintenance costs. The primary reason that bag house filter systems are relatively large is that they are designed to separate the solids from the air stream by reducing the velocity of the air stream to approximately 1-3 feet per minute as the air stream passes through the bag element. As a result, a relatively large surface area is required, and may require a thousand or more large bags in some industrial applications. These bags must be connected by a piping system to deliver the air stream to them, and require frequent attention to keep the operating satisfactorily.
Additional difficulties may arise with bag house filtering systems if the solids to be separated are tacky or sticky. Solids which are not free flowing can plug the air distribution system or the bags themselves. Bag house systems are also very susceptible to fire caused by static charges in the vicinity of oxygen containing air carrying finely divided entrained solids. If the solids contain additional flammable materials, such as hydrocarbon resins, the fire danger is further aggravated. The cost of a bag house system is therefore extremely high, and such a system may not be able to provide safe and continuous operation.
Flat filter elements are also well-known means for continuously removing small particulate solids from an air stream, but their use on an industrial has not always proven acceptable, especially in the removal of sticky resinated particles mentioned above. Problems associated with flat filter elements include designing a filter element having sufficient porosity for maintaining a satisfactory flow rate while at the same time removing small particles. In the case of sticky particles, this problem is particularly difficult to solve. In addition, as solids accumulate and compact on a flat filter element, the throughput of the element is decreased further. If the pores of the filter are enlarged to increase air flow through the element, the small entrained particles are not satisfactorily retained on the filter element.
A need therefore remains for a compact, relatively inexpensive filter system for removing small particles from a relatively high-volume air stream, and which will continuously remove small and relatively sticky particles, such as resinated wood particles, from the air stream.